1. Field of the Invention
The present invention relates to an optical add-drop multiplexer (ADM) and particularly, to an optical ADM apparatus provided with a narrow band-pass optical filter such as a fiber grating.
2. Description of the Related Art
A conventional example of such optical ADM apparatus is shown in FIG. 4 where while a particular wavelength component is extracted from a wavelength division multiplexed (WDM) light signal, an equivalent of the wavelength component extracted is added. As shown, the conventional ADM apparatus comprises a first optical circulator 31, a second optical circulator 32, and a fiber grating 33 connected between the first 31 and the second optical circulator 32. The wavelength division multiplexed (WDM) light signal is introduced to an input optical fiber 31a of the first optical circulator 31 and directed by the operation of the first optical circulator 31 to pass via a signal fiber 31b to the fiber grating 33 where a predetermined wavelength component of the light signal is reflected back. The predetermined wavelength component is returned to the first optical circulator 31 and outputted from its output optical fiber 31c as a drop light. Meanwhile, an add light of which wavelength is identical to that of the drop light is introduced to an input optical fiber 32a of the second optical circulator 32. The add light is directed by the operation of the second optical circulator 32 to pass via a signal fiber 32b to the fiber grating 33 where it is reflected back to the second optical circulator 32. Accordingly, the add light is combined with the light signal less the drop light and delivered from an output optical fiber 32c of the second optical circulator 32.
The conventional ADM apparatus has a drawback that both the drop light directed by the first optical circulator 31 to the fiber grating 33 and the add light directed by the second optical circulator 32 to the fiber grating 33 may incompletely be reflected back hence developing leak components which can pass across the fiber grating 33. For example, the leak component may be as a high level as 30 dB (0.1%) as illustrated in FIG. 5. The drop light and the add light are identical in the wavelength and their leak components may interfere with each other generating a beat noise and declining the transmission characteristics. More specifically, the leak component of 30 dB (S/X=30) leads to a transmission penalty of about 0.8 dB as shown in FIG. 6 which illustrates the dependence of the transmission penalty on the beat noise.